many people have now realised the gains in MPG that can be acchieved through ecu remapping and are having their cars ECU Remapped for this reason alone. some people are gaining upto 10mpg after ecu remapping!
take a look at this article from the sunday times on economy ecu remapping.
http://www.timesonline.co.uk/tol/driving/features/article4030131.ece